1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with suppression of spurious capacitive-coupling signals and to a method for controlling a microelectromechanical gyroscope.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has witnessed an ever-increasing diffusion in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures having at least one mass connected to a fixed body (stator) by springs and movable with respect to the stator according to predetermined degrees of freedom. The movable mass and the stator are capacitively coupled by a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the applied force. Vice versa, by providing appropriate biasing voltages, it is possible to apply an electrostatic force on the movable mass to set it in motion. In addition, in order to obtain electromechanical oscillators, the frequency response of inertial MEMS structures, which is typically of a second-order low-pass type, is exploited. By way of example, FIGS. 1 and 2 show the plot of the magnitude and phase of the transfer function, respectively, between the force applied on the movable mass and the displacement thereof with respect to the stator in an inertial MEMS structure.
Many MEMS (in particular, all electromechanical oscillators and gyroscopes) must be provided with driving devices that have the task of maintaining the movable mass in oscillation.
A first type of known solution envisages applying, in open loop, a periodic force at the resonance frequency of the MEMS structure. The solution is simple, but also highly ineffective, because the resonance frequency is not known precisely since dispersions in the processes of micromachining of semiconductors are not eliminable. In addition, the resonance frequency of each individual device can vary in time, for example on account of temperature gradients or, more simply, aging.
Feedback driving circuits have then been proposed, based upon the use of sigma-delta modulators. Circuits of this type are undoubtedly more effective than the previous ones in stabilizing the oscillation of the movable mass at the actual resonance frequency and in suppression of any disturbance. However, various stages are necessary for filtering, decimation, and further processing of the bitstream supplied by the sigma-delta modulator. For this reason, currently available feedback driving circuits involve a complex production process, are cumbersome, and, finally, costly.
In addition, it is necessary to consider that gyroscopes have a complex electromechanical structure that includes two masses that are movable with respect to the stator and are coupled to one another so as to present one relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonance frequency. The other mass is drawn along in the oscillatory motion and, in the case of rotation of the microstructure with respect to a predetermined axis with an angular velocity, is subjected to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass operates as an accelerometer that enables detection of the Coriolis acceleration.
In principle, the two masses should be electrically insulated from one another. The insulation, however, is never perfect and hence reading of the driven mass can affect the oscillating motion of the mass dedicated to driving, thus disturbing or even preventing proper operation of the gyroscope.